a. Field of the Invention
The instant disclosure relates generally to systems and methods for providing irrigation fluid during cardiac ablation of cardiac tissue. In particular, the instant disclosure relates to families of systems and methods for providing irrigation fluid during cardiac ablation of cardiac tissue in which irrigation fluid is delivered at a first flow rate in a first time period and at a second flow rate in a second time period. The second time period is temporally after the first time period, and the second flow rate is at least half of the first flow rate. The instant disclosure also relates to systems for controlling delivery of power during cardiac ablation of cardiac tissue. In particular, the instant disclosure relates to systems in which energy is delivered at a first power level in a first time period and at a second power level in a second time period. The second time period follows the first time period, and the second power level has a magnitude of at least about (or approximately) one third (˜⅓) of the first power level.
b. Background Art
Electrophysiology catheters are used in a variety of diagnostic and/or therapeutic medical procedures to diagnose and/or correct conditions such as atrial arrhythmias, including for example, ectopic atrial tachycardia, atrial fibrillation, and atrial flutter. Arrhythmias can create a variety of conditions including irregular heart rates, loss of synchronous atrioventricular contractions and stasis of blood flow in a chamber of a heart which can lead to a variety of symptomatic and asymptomatic ailments and even death.
A medical procedure in which an electrophysiology catheter is used includes a first diagnostic catheter deployed through a patient's vasculature to a patient's heart or a chamber or vein thereof. An electrophysiology catheter that carries one or more electrodes can be used for cardiac mapping or diagnosis, ablation and/or other therapy delivery modes, or both. Once at the intended site, treatment can include, for example, radio frequency (RF) ablation, cryoablation, laser ablation, chemical ablation, high-intensity focused ultrasound (HIFU)-based ablation, microwave ablation. An electrophysiology catheter imparts ablative energy to cardiac tissue to create one or more lesions in the cardiac tissue and oftentimes a contiguous or linear and transmural lesion. This lesion disrupts undesirable cardiac activation pathways and thereby limits, corrals, or prevents errant conduction signals that can form the basis for arrhythmias.
During RF ablation, local temperature elevation can result in coagulum formation on the ablation electrode, resulting in an impedance rise. As the impedance increases, more energy is passed through the portion of the electrode without coagulation, creating even higher local temperatures and further increasing coagulum formation and the impedance. Finally, enough blood coagulates onto the electrode that no energy passes into the targeted tissue, thereby requiring the catheter to be removed from the vascular system, the electrode to be cleaned, and the catheter to be repositioned within the cardiac system at the desired location. Not only can this process be time consuming, but it can be difficult to return to the previous location because of the reduced electrical activity in the targeted tissue, which has been previously ablated. Recent studies have also demonstrated the formation of a so-called soft thrombus in RF ablation. The formation of the soft thrombus results from heat induced protein denaturation and aggregation and occurs independently of heparin concentration in serum. In addition, RF ablation can generate significant heat, which, if not controlled, can result in excessive tissue damage, such as tissue charring, steam pop, and the like.
Accordingly, it can be desirable to monitor and/or control the temperature of ablation electrode assemblies. It can also be desirable to use ablation electrode assemblies to provide irrigation fluid during RF ablation. RF ablation catheters can be configured to provide temperature feedback during RF ablation via a thermal sensor such as a thermocouple or thermistor. A temperature reading provided by a single thermal sensor cannot accurately represent the temperature of the electrode/tissue interface. This is because a portion of the electrode that is in direct contact with the targeted tissue can have a higher temperature than the rest of the electrode that is being cooled by the blood flow. The orientation of the RF ablation catheter can affect the position of the thermal sensor, and accordingly, can affect the temperature reading provided by the thermal sensor. If the thermal sensor is in contact with the targeted tissue, the thermal sensor can provide a certain temperature reading generally corresponding to the temperature of the targeted tissue, but if the thermal sensor is not in contact with the targeted tissue, there will be a time lag before the thermal sensor provides a temperature reading generally corresponding to the temperature of the targeted tissue, and due to the cooling effect of the blood flow, the thermal sensor can never approach the actual temperature of the targeted tissue. In an effort to overcome the effect that the orientation of the catheter can have on temperature sensing, multiple thermal sensors positioned at different locations on the electrode can be used. For example and without limitation, the highest measured temperature can be used to represent the electrode/tissue interface temperature. However, temperature measurements provided by multiple thermal sensors cannot always accurately reflect the temperature of the electrode/tissue interface (e.g., heat transfer between the multiple thermal sensors can affect the temperature reading of each thermal sensor).
Accurate, real-time correlation between the temperature recorded at the distal end portion of ablation electrode assemblies and that of the targeted tissue typically becomes more difficult over time. This is because the temperature recorded at the distal end portion of ablation electrode assemblies generally plateaus, whereas the temperature of the targeted tissue continues to steadily rise over time. RF ablation would benefit from an improved correlation between the temperatures of ablation electrode assemblies and targeted tissue, especially toward the end of an RF ablation cycle. RF ablation would also benefit from a cycle configured to increase the total energy delivered during an RF ablation cycle (thereby increasing lesion size), while still sufficiently controlling temperature to avoid adverse effects such as tissue charring.